I've bought 2 dogs from breeders over the years, all with spay/neuter clauses, but none with the threat of suing if the spay/neuter isn't done.
It can be very difficult to find a breeder sometimes, depending on the breed, it can take months to finally find a puppy and then to add the possiblility of being sued if you don't get the dog spayed or neutered by 6 months of age, you'll just be driving some people to backyard breeders and pet stores. I wouldn't sign such a contract, not because I won't get a dog spayed or neutered. I always do, I just prefer to get male dogs done closer to their first birthday. I've never had a problem with getting a puppy from a breeder once I've found one, although I have found some breeders I wouldn't even consider getting a puppy from. Not everyone who is trying to get a puppy is an irresponsible idiot, so if you treat people as such and make getting a puppy as difficult as you can, then they will find alternate resources and that makes the breeders part of the problem as well in my opinion.